A γ′ (gamma prime) phase precipitation strengthened Ni-based alloy, which contains large amounts of alloy elements such as Al and Ti, has been applied to heat-resistant members for aircraft engines and gas turbines for power generation.
In particular, turbine disks among turbine components require high strength and high reliability, to which a Ni-base forged alloy has been applied. The term “forged alloy” is used in contrast to the term “cast alloy”, which is a term for an alloy to be used with its casting solidification structure, and is a material produced by a process of hot working an ingot obtained by melting and solidification so that the ingot has a desired shape of a component. Due to the hot working, a coarse and heterogeneous cast and solidified structure is turned into a fine and homogeneous forged structure, and thereby mechanical characteristics such as the tensile strength and fatigue properties are improved. However, if too many γ′ phases that are a strengthened phase exist in the structure, it may become difficult to carry out hot working represented by press forging, which may cause defects during production. In order to prevent this, the content of components of the composition of a forged alloy, such as Al and Ti, which contribute to the strengthening, is generally more limited than that in a cast alloy that is not subjected to hot working. Udimet 720 Li (“Udimet” is a registered trademark of Special Metals Corporation) can be mentioned as a turbine disk materials having the highest strength at the present, and in the material, the amounts of Al and Ti are 2.5% by mass and 5.0% by mass, respectively.
To improve the material strength, a process of producing a Ni-based alloy by using a powder metallurgy method has been implemented, instead of a conventional process of melting an ingot. According to this method, the alloy composition can include a larger amount of above-described strengthening elements compared with an alloy obtained by a melting and forging method. However, to prevent contamination by impurities, it is inevitable to perform high-level management of the production processes, and thus, the production costs may be high, and therefore, this production method is used for limited purposes.
As described above, forged alloys used in turbine disks have a great problem of simultaneously realizing high strength and high hot workability, and thus, alloy compositions and production methods that can solve this problem have been developed.
For example, WO 2006/059805 A discloses an extremely strong alloy that can be produced by a conventional melting and forging method. A composition of this alloy contains a larger amount of Ti than the composition of Udimet 720 Li and additionally contains a large amount of Co, thus enhancing the stability of its structure and also enabling it to be hot worked.
There has been another attempt to improve the hot workability by a production method. “Proceedings of the 11th International Symposium on Superalloys” (TMS, 2008), pp. 311-316, discloses an experimental report regarding a forged member of Udimet 720 Li, in which the hot workability is more improved as a cooling rate decreased when the material was cooled from a raised temperature of 1,110° C.